Trona as found in the Green River area of Wyoming consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O). A typical analysis of trona contains:
______________________________________ Constituent Percent ______________________________________ Na.sub.2 CO.sub.3 45 NaHCO.sub.3 36 H.sub.2 O 15.30 NaCl 0.04 Na.sub.2 SO.sub.4 0.01 Fe.sub.2 O.sub.3 0.14 Organic Matter 0.30 Insolubles 3.20 ______________________________________
In addition to a water insoluble fraction, which amounts to about 3%, resulting from the association of the trona with shale stringers or beds in the trona deposit, organic matter in the order of about 0.3% is present which would contaminate the desired product, e.g. sodium carbonate precursor crystals, unless it is removed. It is believed that the organic matter in the trona consists of kerogenaceous material containing monocarboxylic acids, dicarboxylic acids, certain unsaturated acids, steroids, and certain resin acids. The presence of these organics, as well as organic impurities picked up in the circulating liquors in the plant are present in the solution to be crystallized to obtain the desired sodium carbonate precursor crystals.
Various methods previously have been proposed to reduce the organics to an acceptable level so as not to adversely affect the crystal quality. For instance in U.S. Pat. Nos. 2,962,348 and 3,131,996 the crude trona is decarbonized at relatively high tempertures, i.e. in the order of about 400.degree. C. to about 800.degree. C. At these temperatures the organic matter present in the crude trona is substantially removed by oxidation and distillation. This method, of course, would involve a high heat requirement, thus increasing the cost of the ultimate product. Also, heating the trona in excess of 400.degree. C., while eliminating a substantial amount of the carbon of the crude trona, substantially increases the quantity of water soluble silicate in the crude sodium carbonate, the removal of which is difficult, requiring the bleeding off of substantial amounts of the mother liquor separated from crystallizers. This results in a loss of valuable carbonate values in the discarded mother liquor.
Another method proposed for the reduction of organics is that described in U.S. Pat. No. 3,028,215, wherein the sodium carbonate precursor crystals containing organic impurities are subjected to a high temperature calcination, i.e. temperatures in the order of 400.degree. C. to 800.degree. C., wherein the organic impurities are either volatilized or converted to a coke which allegedly does not present any problem in the utilization of the resulting soda ash in commerce. However, the crystal quality of the precursor crystals is not significantly improved because the organics are present during the crystallization at which time their adverse effect is realized on the quality of the crystals produced during the crystallization. Also, the equipment and heating requirements to calcine the contaminated sodium carbonate precursor crystals are significant, particularly when production rates are of the magnitude of more than 1000 tons of product per day.
Still another method for reducing organics is that described in U.S. Pat. No. 3,260,567, wherein a crude trona is first converted to crude sodium carbonate at relatively low temperatures, i.e. in the order of 150.degree. C. to 200.degree. C., and then the crude sodium carbonate is dissolved in an aqueous solution. After removal of the insolubles by clarification, the organics are substantially reduced by passing a solution of crude sodium carbonate through a bed of an adsorbent, such as activated carbon, prior to crystallization. Obviously, after a number of filtrations through the carbon bed have taken place, the capability of the carbon to remove these organics from the carbonate process solution is substantially reduced. The prior practice, as disclosed in this patent, for the removal of these adsorbed organics from the activated carbon used in the purification of carbon process solutions has been achieved by reactivation of the carbonate by heating it in special ovens to temperatures in excess of about 500.degree. C., preferably from about 700.degree. C. to 900.degree. C. The adsorbed organic contaminants are pyrolyzed to carbon or removed as products of combustion, thereby restoring the adsorption activity of the carbon to its original state. However, to subject the carbon to a heat treatment, the carbon first must be removed from the adsorption apparatus and after regeneration returned to the same. This requires a large amount of work and considerable loss of time. Also, there is a normal attrition loss of the carbon being reactivated, which is estimated to be on the order of about 5% to 10%. Furthermore, the ovens in which the carbon is to be regenerated are expensive. These drawbacks in the prior art method of regenerating carbon beds which remove organic contaminants from carbonate process solution add to the overall cost of the desired product.
Still another method is found in U.S. Pat. No. 3,528,766, which discloses the removal of soluble organic impurities from trona solutions using activated carbon and regeneration of the activated carbon with water which is maintained at a temperature substantially above the temperature of the solution from which the organics are removed.
It has now been found that the activated carbon which has become deactivated as a result of adsorbing organic contaminants from carbonate process solution may be regenerated by a relatively simple procedure which comprises passing a heated aqueous solution through the activated carbon bed, thereby desorbing essentially all of the organic contaminants adsorbed thereon. With the present wash method it is not necessary to remove the carbon from the adsorption towers. The washing can be accomplished in situ in the same column. By providing a group of columns arranged in parallel it is possible to operate continuously. Upon exhaustion of the adsorptive capacity of a given column or upon reduction of its adsorbent capacity to an uneconomical low level, e.g. absorption efficiencies reduced to about 75% or less, preferably to a level of about 50% or less, of the original adsorptive capacity of the adsorbent prior to the absorption cycle, the carbon tower can be taken off-stream and the deactivated carbon regenerated. Regeneration with the hot aqueous wash can be effected at temperatures from about 50.degree. C. to about 95.degree. C., preferably about 60.degree. C. to 90.degree. C., with temperatures in the range of about 70.degree. C. to 85.degree. C., being especially preferred.
The present invention is directed to the finding that regeneration of the exhausted activated carbon to a high level of adsorptive capacity can be achieved by passing the aqueous wash liquor through the bed being regenerated at a flow rate such that the activated carbon particles are periodically "reoriented" or "fluffed" at random intervals. That is, by adjusting the flow rate of the aqueous wash liquor through the bed of activated carbon the carbon particles become reoriented and portions of the activated carbon are exposed to aqueous wash liquor which may have adsorbed organics thereon and which are normally difficult to remove due to the degree of compaction and preferential channeling during the desorption cycle. The organics then can be more easily desorbed so that at least 60%, preferably at least 70% of the original activity of the adsorption capacity of the carbon is restored. By varying the flow rates of the aqueous wash liquor through the carbon bed at random intervals from flow rates insufficient to effect reorientation of the carbon bed to flow rates sufficient to achieve a state of "incipient fluidization" up to flow rates sufficient to effect an expansion of the carbon bed volume of about 75% of the static bed volume, it has been found that the carbon particles become reoriented, thereby permitting exposure of the carbon to the aqueous wash liquor which desorbs the organic contaminants from the carbon.
By the phrase "incipient fluidization" is meant the state of the carbon bed particles which become separated due to the flow rate of the wash liquor so that no stable arrangement of the particles exists and the particles vibrate or circulate locally in a semi-stable arrangement to a point wherein fluidization begins and particles reorientation occurs. Flow rates which achieve incipient fluidization up to about a bed expansion of about 50% of the static bed volume are especially preferred to minimize loss of the carbon carried over in the wash liquor. Superficial velocities of the wash liquor necessary to produce reorientation of the carbon bed particles will be dependent upon a number of variables, such as the size, distribution, density, true porosity and fractional voids of the carbon particles composing the bed; as well as the nature of the fluidizing media, its temperature, density, viscosity, pressure and other related parameters. The flow rate to achieve reorientation for a given bed of carbon can be best determined by test in equipment where visual observation of the bed can be made. For instance, superficial velocities of the wash liquor in the order of about 1 to about 40, preferably flow rates up to about 30 gallons per minute per square foot of cross section area of the carbon bed generally can be employed to achieve reorientation of the carbon bed particles. Depending upon the carbon bed particles employed, flow rates of the aqueous wash liquor insufficient to achieve substantially any reorientation may vary from about 0.25 to about 25, preferably from about 0.5 to about 12, gallons per minute per square foot of cross section area of the carbon bed.
By maintaining the flow rates of the wash liquor at a level which reorients or fluffs the carbon particles, regeneration efficiency can be maintained at a relatively high level in the order of about 80% or more desorption of the adsorbed organic impurities.
The adsorbed organic contaminants may be desorbed from the carbon by a procedure which utilizes previously employed wash liquors in a sequence of wash steps permitting an economical use of wash liquor. For instance, an aqueous wash liquor containing organic contaminants previously desorbed in a prior wash step may be used as the first wash liquor in a new cycle to desorb from the activated carbon the easily removable organic contaminants and thereafter subsequently employing a second aqueous wash liquor which has a substantially lower organic contaminant level than the first wash liquor. This second wash liquor may also previously have been employed in a prior wash cycle or it may be substantially free of organic contaminants, e.g. a fresh or relatively pure aqueous solution. As long as there is some capacity of the aqueous wash liquor to desorb the adsorbed organic contaminants from the activated carbon, such wash liquor may be employed in a wash cycle of the process of the present invention.
If this sequential wash procedure is used the first aqueous wash liquor employed is a relatively impure aqueous wash liquor which previously may have been used as the second wash liquor in desorbing organic contaminants from a deactivated carbon bed. The first wash liquor has an organic contaminant level lower than the carbonate process solution. The second aqueous wash liquor employed has a lower organic contaminant content than the first aqueous wash liquor in order to further desorb the organics from the carbon. This second aqueous wash liquor preferably is a third wash liquor previously employed in a prior wash cycle. After this second wash a third aqueous wash liquor substantially free of organic contaminants is employed. For instance, if a carbonate process solution contains an organic contaminant level in the order of about 370.+-. 30 parts per million, the organic contaminant level in the first aqueous wash liquor should contain a maximum of about 300 parts per million; in the second aqueous wash liquor a maximum level of about 150 parts per million; and in the third aqueous wash liquor a maximum level of about 25 parts per million. However, since the organic contaminant level of the carbonate process solution may vary, as indicated above, the maximum organic contaminant levels in each of the aqueous wash liquors may likewise vary.
With a three wash liquor wash cycle, desorption of organic contaminants from the activated carbon has been found to exceed 60% of the originally adsorbed, preferably desorption in the order of more than 70% is achieved. A plurality of such washing liquors may be employed to regenerate the activated carbon bed to a level which meets the needs of a particular plant capacity to cut down the number of off cycles for the carbon columns.
Suitable sources of the aqueous wash liquor include plant condensate and make-up water, such as from a local stream, spring or river which has been suitably treated to prevent the precipitation of insoluble matter upon contact with carbonate containing solutions. These may contain dissolved therein minor amounts of sodium carbonate precursor crystals, e.g. an aqueous solution containing about 1% sodium carbonate values. While other chemical components, e.g. alkali metal carbonates and hydroxides, may be employed in the aqueous wash liquors, it has been found that these components offer no significant improvement over the use of aqueous wash liquors maintained at elevated temperatures.
By the phrase "carbonate process solution" is meant a substantially saturated aqueous solution from which the sodium carbonate precursor crystals, i.e. sodium bicarbonate, sodium sesquicarbonate, anhydrous sodium carbonate and sodium carbonate monohydrate, may be formed as the stable crystal phase in a subsequent crystallization procedure. In addition to trona as the source of the carbonate process solution, other natural minerals may be used such as nahcolite (NaHCO.sub.3), thermonatrite (Na.sub.2 CO.sub.3.H.sub.2 O), natron (Na.sub.2 CO.sub.3 .10H.sub.2 O) and dawsonite (NaAlCO.sub.3 (OH).sub.2), particularly when these minerals are associated with or near kerogen type deposits. The carbonate process solution may be prepared by a variety of different prior art methods described in U.S. Pat. Nos. 2,343,080; 2,343,081; 2,639,217; 2,704,239; 2,770,524; 2,962,348; 3,028,215; 3,131,996 and 3,260,567.